The present disclosure relates to an ignition coil assembly for ignition of a spark plug, such as in an automobile. A known ignition assembly 10 for ignition of an automobile spark plug 11 is shown in prior art FIG. 1. The assembly 10 connects to the spark plug 11 via a boot 12 connected with an ignition coil assembly 13 having a steel laminated core 14 and a molded plastic cover 15. The molded plastic cover 15 also houses a connector/electrical harness 16 at one side of the coil assembly 13.
As shown in prior art FIG. 2 with a portion of the cover 15 removed, a primary coil 17 is surrounded by a secondary coil 18 around what is known as an I core portion 19 adjacent ends of a C core portion 20 forming core 14. The I core portion 19 and the C core portion 20 are each formed of a stack of individual electrical steel laminations 19A and 20A, respectively.
A magnet 21 is inserted between one leg of the C core portion 20 and a side adjacent one end of the I core portion 19.
The laminations comprise electrical steel and the primary and secondary coils comprise copper magnet wire. An automotive battery provides a nominal 12 volt DC supply to the primary coil 17 as an inner winding around the I coil portion 19. The number of copper turns are on the order of a few hundred for the primary coil, whereas the surrounding secondary coil 18 has several thousand turns of magnet wire. A diameter of the primary coil wire is larger as compared to a diameter of the secondary coil wire. Both primary and secondary coils are connected in series.
The coil assembly 13 of the overall ignition assembly 10 works together with a number of other components that make up the ignition system. The modern day system utilizes sensors placed around the engine in order to provide signals to the ECM (Electronic Control Module—also known as the computer). The ECM then determines where the engine crank and camshafts are in mechanical degrees. When the timing is right, the voltage is sent to the ignition primary coil which will then be transferred to the secondary coil. This high voltage is sent to the individual spark plug at the moment the engine piston is at the top of the stroke. The fuel and air mixture is then burned and the piston is forced downward. This downward stroke partially rotates the crankshaft which supplies power to the transmission. The transmission then converts this power to torque which is delivered to the wheels for mobility. This action repeats for every ignition coil and piston in the engine in timed intervals. The faster the automobile drives, the faster the ignition process.
When a switch is closed, current will build easily in the primary coil 17 because it has less inductance than the secondary coil 18. Because the primary coil current is direct current, it creates a very small magnetic field with a constant magnitude which causes a minimal magnetic flux to flow in the steel laminations 19A and 20A. When the switch is opened, the current potential across the primary coil 17 collapses and dumps its energy into the secondary coil 18 which creates a much higher potential due to the many more turns of copper. Therefore this change of energy will add much more magnetic flux in the laminations. This exchange of energy happens within a few milliseconds. As the boost of current from the primary coil 17 decays down to zero, the magnetic flux completes a magnetic circuit loop and causes a large voltage potential across the secondary coil 18. This secondary coil voltage potential is typically in a range between 40,000 to 70,000 volts depending upon the type of engine it was designed for. It also maintains the maximum amount of voltage potential transfer. This high potential then has the ability to jump across a gap 11A of the spark plug 11 in order to ignite a fuel and air mixture in the cylinder at top dead center forcing the piston downward and thus causing a stroke which will help rotate the crankshaft of the internal combustion engine.
Design criteria requirements for an ignition coil are:
(1) Maximum transfer of energy without excess losses in the laminations. This is because losses are wasted energy and are converted to heat. Efficiency is reduced and the useable life of the product is compromised.
(2) Minimum decay time for transfer of energy. This is necessary for two reasons. First, the quicker the transfer of energy, the lower the losses on all the components. Secondly, tighter EPA (Environmental Protection Agency) emission regulations require recirculating the unburned gas mixture known as hydro carbons. These hydro carbons are not as easy to ignite as the pure fuel in the initial burn. Therefore the spark across the spark plug gap must have the highest potential.
Modern automobile ignition systems have advanced from the early days of one single ignition coil and distributor assembly for all cylinders to individual ignition coils, having one per cylinder, such as the single coil assembly 13 shown in FIG. 1. Different core geometric designs have been used such as the C core portion 20 and the I core portion 19 shown in FIG. 2, and C core portion 7 and T core portion 22 as shown in prior art FIG. 4 also having primary and secondary coils 8 and 9 respectively. However, whether it is C and I or C and T core portions, the function remains the same. The basic building blocks are also the same including the introduction of a permanent magnet 21 shown in FIG. 2 and also shown in the cross sectional view of FIG. 3. In the C and T arrangement the permanent magnet 23 is disposed between a T end of the T core portion 22 and a side of one leg of the C core portion 7. An example of the prior art magnet used in a C and T core portions is U.S. Pat. No. 5,241,941.
The advantage of the magnet 21 in FIG. 3 or the magnet 23 in FIG. 4 is a magnetic flux source within the steel core magnetic circuit. The permanent magnet flux opposes the previously described weak flux generated from the primary coil 17 when the switch is closed. This is to ensure that no voltage is generated on the secondary coil 18 until the switch is opened. At this moment the maximum voltage transfer is made.